Mechanical face seals are commonly used to provide a seal between a stationary housing and a rotating shaft. Such seals include a rotating ring, or rotor, mounted on the shaft and a stationary ring, or stator, mounted on the housing. Either the stator or the rotor is biased toward the other to provide a biased seal therebetween.
A typical seal design for inhibiting process fluid, whether liquid or gas, from escaping from a housing along a rotating shaft includes two seals in-fluid communication with an intermediate chamber containing a buffer fluid. One seal radially pumps the buffer fluid having a certain pressure across the seal between a stator and rotor into the housing containing the process fluid. The process fluid in the housing has a lower pressure than the buffer fluid in the intermediate chamber. The other seal radially pumps the buffer fluid to an environment external to the housing, such as ambient, which is at a pressure lower than the buffer fluid in the intermediate chamber.
To accomplish this radial pumping, each seal includes spiral grooves on either the face of the stator or rotor. The grooves are angled relative to the radius and circumference of the rotating shaft, and when the rotor is rotating, the grooves radially pump the buffer fluid across the seal. The rotor must be rotated at a speed sufficient to generate a lift force that overcomes the hydraulic and mechanical forces biasing the rotor and stator toward each other in order to create a gap between the rotor and stator through which the buffer fluid is pumped. This pumping of the high-pressure buffer fluid toward the lower-pressure external environment or process fluid inhibits the loss of the process fluid from the housing. U.S. Pat. No. 5,375,853 discloses a seal design of this type.
In another design, grooved face seals are used in pumps to provide a seal between a high-pressure gas (e.g., a combustible gas) and the ambient atmosphere. In this situation, two seals are commonly used. A grooved inner seal radially pumps the high pressure gas to an intermediate chamber, and a grooved outer seal radially pumps from the intermediate chamber to the atmosphere. The intermediate chamber routes the high-pressure gas to a flare stack where the pumped gas is burned. The amount of high-pressure gas that is lost through the outer seal is thereby minimized. An example of this type of seal is disclosed in U.S. Pat. No. 5,217,233.
These types of seals are also used in aerospace applications, such as disclosed in U.S. Pat. Nos. 5,941,532 and 6,257,589, which are assigned to the assignee of the present application and fully incorporated herein. When used in an aerospace application, these seals have a limited operating window in which they are effective. At low surface speeds and high altitude conditions (with very low ambient pressures) it has been discovered that the seals may not generate sufficient lift to overcome hydraulic and mechanical closing forces, resulting in contact between the sealing faces and degraded performance. A need exists for seals which can more effectively operate in low surface speeds/high altitude conditions.